笛卡儿坐标系的双多普勒天气雷达三维风场反演技术

文中研究了笛卡尔坐标系下双多普勒天气雷达三维风场反演技术 ,提出了包括雷达原始资料的预处理 ,空间插值以及可靠性检验的新方法 ,提高了反演结果的可信度和精确度。使用模拟的双多普勒雷达体扫资料进行了反演试验 ,结果表明 :本文的方法能够比较真实地反映风场的三维结构 ,可以用于真实风场的反演。     

Wind energy estimates by use of a diagnostic model

A diagnostic model is a relatively simple and practical tool for modeling the wind flow of the boundary layer in complex terrain. The model begins with a wind analysis based on available surface wind reports and geostrophic winds (computed from pressure data). The height of the boundary layer top (upper surface of the computational domain) is prescribed to fit local conditions. Using the continuity equation in terrain-following coordinates, the winds at mesh points are adjusted to produce nondivergence while maintaining the original vertical component of vorticity. The method of computing the nondivergent winds uses direct alterations. This method may be useful for other modeling purposes and will be described. Data for a long period (usually a year) are analyzed to obtain eigenvectors and the associated time series of their coefficients at each observation time. The model is run only for the five or six eigenvectors that explain most of the variance. The wind field at any particular time is reconstructed from the eigenvector solutions and their appropriate coefficients. Comparisons of model results with measured winds at sites representing different types of terrain will be shown. The accuracy and economy of the model make it a useful tool for estimating wind energy and also for giving wind fields for low-level diffusion models.     

地面风诊断模式的数值试验和模拟

本文用一个简化的一层地面风数值诊断模式，在台湾岛及海峡地区对6个个例进行了数值试验。结果表明，假定的地形影响层较厚时，地面流场中台湾岛周围的流线更趋于与地形线平行，地形效应更明显；地表粗糙度增大时，地面风向逆时针偏转，风速减小；考虑非绝热加热后，地面流场中分流现象明显减弱，并在山脊附近形成辐合线，同时岛后辐合减弱甚至消失；而地形高度的动力作用则是分流、绕流和岛后辐合等现象的内在根源。在上述研究基础上，对同样个例进行了数值模拟。结果在一定程度上反映了实际风场。     

The combined use of three different approaches to obtain the best estimate of meso-β surface winds over complex terrain

Three approaches for estimating meso-β (Δx=5–10 km) surface winds over complex terrain are applied to obtain best estimates of typical summertime surface flow in Israel, based on a detailed 3-D model, a one-level sigma model and a dense network of surface wind observations. A scheme for combining the three approaches is outlined and illustrated through two examples showing how the approaches complement each other. It is suggested that such a man-machine combination is best for estimation of surface winds over complex terrain.     

Dynamical and Microphysical Retrieval from Simulated Doppler Radar Observations Using the 4DVAR Assimilation Technique

Based on a cloud model and the four-dimensional variational (4DVAR) data assimilation method developed by Sun and Crook (1997), simulated experiments of dynamical and microphysical retrieval from Doppler radar data were performed. The 4DVAR data assimilation technique was applied to a cloud scale model with a warm rain parameterization scheme. The 3D wind, thermodynamical, and microphysical fields were determined by minimizing a cost function, defined by the difference between both radar observed radial velocities and reflectivities and their model predictions. The adjoint of the numerical model was used to provide the gradient of the cost function with respect to the control variables. Experiments have demonstrated that the 4DVAR assimilation method is able to retrieve the detailed structure of wind, thermodynamics, and microphysics by using either dual-Doppler or single-Doppler information. The quality of retrieval depends strongly on the magnitude of constraint with respect to the variables. Retrieving the temperature field, cloud water and water vapor is more difficult than the recovery of the wind field and rainwater. Accurate thermodynamic retrieval requires a longer assimilation period. The inclusion of a background term, even mean fields from a single sounding, helped reduce the retrieval errors. Less accurate velocity fields were obtained when single-Doppler data were used. It was found that the retrieved velocity is sensitive to the location of the retrieval domain relative to the radars while the other fields have very little changes. Two radar volumetric scans are generally adequate for providing the evolution, although the use of additional volumes improves the retrieval. As the amount of the observations decreases, the performance of the retrieval is degraded. However, the missing observations can be compensated by adding a background term to the cost function. The technique is robust to random errors in radial velocity and calibration errors in reflectivity. The boundary conditions from the dual-Doppler synthesized winds are sufficient for the retrieval. When the retrieval is mainly controlled by the observations in the regions away from the boundaries, the simple boundary conditions from velocity azimuth display (VAD) analysis are also available. The microphysical retrieval is sensitive to model errors.     

Radiative Processes in the Stable Boundary Layer: Part II. The Development of the Nocturnal Boundary Layer

The interaction between radiation and turbulence in the stable boundary layer over land is explored using an idealized model, with a focus on the surface layer after the evening transition. It is shown that finer vertical resolution is required in transitional boundary layers than in developed ones. In very light winds radiative cooling determines the temperature profile, even if similarity functions without a critical Richardson number are used; standard surface similarity theory applied over thick layers then yields poor forecasts of near-surface air temperatures. These points are illustrated with field data. Simulations of the developing nocturnal boundary layer are used to explore the wider role of radiation. Comparatively, radiation is less significant within the developed stable boundary layer than during the transition; although, as previous studies have found, it remains important towards the top of the stable layer and in the residual layer. Near the ground, reducing the surface emissivity below one is found to yield modest relative radiative warming rather than intense cooling, which reduces the potential importance of radiation in the developed surface layer. The profile of the radiative heating rate may be strongly dependent on other processes, leading to quite varied behaviour.     

Development of a hurricane boundary-layer wind model

Summary Hurricanes cause a variety of damage due to high winds, heavy rains, and storm surges. This study focuses on hurricanes’ high winds. The most devastating effects of sustained high winds occur in the first few hours of landfall. During the short period, hurricanes’ rainfall often increases, while the low-level pressure gradients continue to weaken. Latent heating does not appear to strengthen the surface winds. The indicator is that dry mechanisms such as the boundary layer processes and terrain are responsible for the damaging winds in the coastal areas. In this study, the design of a dry hurricane boundary layer wind model is described. The goal is to develop a forecast tool with near-real time applications in expeditious wind damage assessment and disaster mitigation during a hurricane landfall event. Different surface roughness lengths and topographic features ranging from flat land to the mountainous terrain of Taiwan were used in the model simulation experiments to reveal how the coastal environment affected the hurricane surface winds. The model performed quite well in all cases. The experiments suggested that the downward transfer of high momentum aloft played a significant role in the maintenance of high wind speeds at the surface. The surface wind maximums were observed on the lee sides of high terrain. The surface streamline analyses showed that the high mountains tended to block the relatively weak flow and caused small eddies, while they forced the stronger flow to turn around the mountains. Due to great difficulty in data collection, the hurricane boundary layer over land remains one of the least understood parts of the system. The dry model proves to be an effective way to study many aspects of hurricane boundary layer winds over a wide range of terrain features and landfall sites. The model runs efficiently and can be run on a medium-size personal computer. Received March 16, 2001 Revised September 10, 2001     

多普勒雷达资料4DVAR同化反演的模拟研究

利用Sun等建立的同化模式和四维变分同化方法对多普勒雷达资料反演大气风场、热力场和微物理场进行了模拟试验研究.反演的基本思路是将4DVAR同化方法应用到三维云模式,定义价值函数表征雷达资料与模式预报结果之间的差别,通过极小化价值函数得到反演场,价值函数相对模式控制变量的梯度由伴随模式求取.试验结果表明,4DVAR同化技术能够从单(双)多普勒雷达资料反演大气三维风场、热力场和微物理场.各个变量反演精度高低与同化过程中变量受约束的大小程度呈正相关.速度场和雨水场反演精度较高,温度场、云水和水汽的反演精度次之,温度场的准确反演需要较长的同化时间.价值函数中加入背景场,哪怕是单点探空给出的平均场信息也有利于提高反演精度.在采用单部多普勒雷达资料进行反演时,速度场的反演误差较大.反演区相对雷达站的位置变化对速度场反演结果有一定的影响,而对其他变量的反演影响很小.两个时次的雷达观测资料基本足够提供反演所需的时间演变信息,同化更多时次的雷达资料,反演效果改进很小.雷达观测资料的缺值会显著降低同化效果,甚至可能导致同化失败,引入背景场可以改善这一状况.4DVAR同化技术对于雷达观测资料误差不太敏感.利用双多普勒雷达合成风场提供水平风场边界条件是比较准确可靠的.在反演主体离边界较远时,VAD风场也基本可用作水平风场边界条件.微物理场的反演对模式中的微物理参数化方案较敏感.     

逐时云迹风资料同化对暴雨预报的模拟试验

文中基于不同的云迹风同化方案,用GRAPES模式对2005年7月11-12日长江中下游一次暴雨强降水过程进行了云迹风资料同化试验及数值模拟,通过对比分析不同方案所得的分析场及预报场的差异,研究逐时云迹风资料三维变分同化对分析场及暴雨预报的影响.首先,根据连续性原理及双通道各层次云迹风资料的误差分析,分3个步骤对7月11日00:00-12:00 UTC共12个时次的双通道云迹风资料进行了初步的质量控制;然后,把经过质量控制的云迹风资料放入基于GRAPES 3D-VAR三维变分同化方案开发的逐小时循环同化系统中进行同化,将得出的分析场与单一时次未经质量控制的云迹风资料同化得出的分析场进行对比,探讨了逐时云迹风资料同化对数值预报分析场的影响;最后,把同化后的分析场作为初始场,用 GRAPES模式对 2005年7月11-12日长江流域暴雨过程做24 h降水预报试验,分析两个同化方案所模拟得到的预报场的差异.结果表明,经资料的筛选、同经纬度单点通道的选择及资料的稀疏化3个步骤控制后,各层次云迹风资料的误差有明显减小;加入经质量控制的逐时云迹风资料其三维变分同化可以提高分析场中风压场及水汽场的质量;而且在暴雨预报试验中可以相对更准确地预报暴雨落区及雨强.     

Three-Dimensional Wind Field Retrieved from Dual-Doppler Radar Based on a Variational Method: Refinement of Vertical Velocity Estimates

In this paper,a scheme of dual-Doppler radar wind analysis based on a three-dimensional variational method is proposed and performed in two steps.First,the horizontal wind field is simultaneously recovered through minimizing a cost function defined as a radial observation term with the standard conjugate gradient method,avoiding a weighting parameter specification step.Compared with conventional dual-Doppler wind synthesis approaches,this variational method minimizes errors caused by interpolation from radar observation to analysis grid in the iterative solution process,which is one of the main sources of errors.Then,through the accelerated Liebmann method,the vertical velocity is further reestimated as an extra step by solving the Poisson equation with impermeable conditions imposed at the ground and near the tropopause.The Poisson equation defined by the second derivative of the vertical velocity is derived from the mass continuity equation.Compared with the method proposed by O’Brien,this method is less sensitive to the uncertainty of the boundary conditions and has better stability and reliability.Furthermore,the method proposed in this paper is applied to Doppler radar observation of a squall line process.It is shown that the retrieved vertical wind profile agrees well with the vertical profile obtained with the velocity–azimuth display(VAD)method,and the retrieved radial velocity as well as the analyzed positive and negative velocity centers and horizontal wind shear of the squall line are in accord with radar observations.There is a good correspondence between the divergence field of the derived wind field and the vertical velocity.And,the horizontal and vertical circulations within and around the squall line,as well as strong updrafts,the associated downdrafts,and associated rear inflow of the bow echo,are analyzed well.It is worth mentioning that the variational method in this paper can be applied to simultaneously synthesize the three-dimensional wind field from multiple-Doppler radar observations.     

A model for computing small-scale wind variations over a water surface

Based on the primitive equations, a one-level model is described for computing surface winds under the meso-scale influences of orography, friction and heating. The effects of atmospheric stability and land-water contrasts are examined in detail. Results of testing the model with data collected during the International Field Year for the Great Lakes (IFYGL) in 1972 are given. The model is able to simulate land and lake breezes as well as meso-scale effects due to gradients in surface water temperature. Compared to simple methods of computing surface winds, use of the model reduces the vector error in estimates of surface winds by about 1.25 m s^–1 on the average.     

正压原始方程变分初值试验

本文利用地转风关系及运动方程作为动力约束条件,对初始风场及高度场进行了调整。在调整场与实测场均方差最小,总平均动能损失也最小的条件下选择了权重因子。还成功地进行了局部风场“嵌入”即所谓“窗户”区试验。对于变分约束对扰动的影响作了初步的探讨。最后还做了24小时预报并与地转初值预报进行了比较。     

Study on Retrieving Three-Dimensional Wind Using Simulated Dual-Doppler Radar Data in the Cartesian Space

1. Introduction Armijo (1969) proposed the three-dimensional wind retrieved equations using multiple-Doppler weather radar network in Cartesian space. There are many sources of error for recovering 3D wind using these equations. These factors include advection and turbulence in the weather system, the error of the empirical relationship between the radar reflectivity factor and the terminal fall velocity of the precipita- tion particle, integrating the air mass continuity equa- tion (problem …     

Nocturnal stable boundary layer height model and its application

A model of the evolution of the nocturnal stable boundary layer height, based on the heat conservation equation for a turbulent flow, is presented. This model is valid for nights with weak winds and little cloudiness in rural areas. The model includes an expression of vertical profile of potential temperature within the boundary layer, which is obtained using micrometeorological information from Prairie Grass, Wangara and O'Neill Projects. The expression turned out to be a second-grade polynomial of the dimensionless height of the nocturnal stable boundary layer. The resulting model is a function of the Monin–Obukhov length, the surface potential temperature of air and the roughness length. This model was satisfactorily compared with micrometeorological data. It was applied at three stations of Argentina, using surface hourly meteorological information. From the results that were obtained, the monthly average values of the stable boundary layer thickness were analysed. The maximum monthly average values occur during the cold season and the minimum ones take place during the hot season. It was observed that the monthly average thickness increases with latitude.     

Low-Level Wind Forecast over the La Plata River Region with a Mesoscale Boundary-Layer Model Forced by Regional Operational Forecasts

A mesoscale boundary-layer model (BLM) is used for running 12-h low-level wind forecasts for the La Plata River region. Several experiments are performed with different boundary conditions that include operational forecasts of the Eta/CPTEC model, local observations, as well as a combination of both. The BLM wind forecasts are compared to the surface wind observations of five weather stations during the period November 2003–April 2004. Two accuracy measures are used: the hit rate or percentage of cases with agreement in the wind direction sector, and the root-mean-squared error (RMSE) of the horizontal wind components. The BLM surface wind forecasts are always more accurate, since its averaged hit rate is three times greater and its averaged RMSE is one half smaller than the Eta forecasts. Despite the large errors in the surface winds displayed by the Eta forecasts, its 850 hPa winds and surface temperature forecasts are able to drive the BLM model to obtain surface winds forecasts with smaller errors than the Eta model. An additional experiment demonstrates that the advantage of using the BLM model for forecasting low-level winds over the La Plata River region is the result of a more appropriate definition of the land–river surface temperature contrast. The particular formulation that the BLM model has for the geometry of the river coasts is fundamental for resolving the smaller scale details of the low-level local circulation. The main conclusion of the study is that operational low-level wind forecasts for the La Plata River region can be improved by running the BLM model forced by the Eta operational forecasts. L. Sraibman and G. J. Berri—Members of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) of Argentina. An erratum to this article can be found at     

双雷达风场反演拼图在登陆台风“莫兰蒂”(1614)强降水精细预报中的同化应用试验

利用福建龙岩、漳州、泉州新一代多普勒天气雷达和厦门海沧双偏振雷达探测资料,采用动态地球坐标系下双雷达三维风场反演与拼图技术,基于天气研究和预报模式（Weather Research and Forecasting,WRF）及其资料同化系统,对登陆台风“莫兰蒂”（1614）引起的2016年9月14—15日福建强降水过程进行了双雷达风场反演拼图资料检验及其三维变分同化对强降水精细预报影响的数值试验,结果发现:（1）动态地球坐标系下双雷达反演风场能合理反映实际风场分布状况,其误差相对较小。相较厦门翔安风廓线雷达及厦门探空秒级测风数据,反演风风向（风速）平均绝对误差分别为7.8°（2.6 m/s）及3.4°（1.1 m/s）;（2）反演风场水平方向稀疏化对同化及预报结果极为重要,过密的反演风场资料会给同化及预报结果带来负效果。文中采用18、6、2 km 3重嵌套,在3重嵌套区域均进行同化以及仅在2 km区域进行同化两种情况下,均表现为当反演风场资料水平分辨率提高到0.1°时,同化分析及预报的台风环流开始受到负影响;且当反演风场资料水平分辨率越高时,负效果越明显。敏感性试验结果显示,分辨率取0.2°时数值预报效果最好;（3）以美国国家环境预报中心全球预报系统（National Centers for Environmental Prediction/Global Forecast System,NCEP/GFS）0.5°×0.5°分析场为初值,基于3个不同起报时刻（2016年9月14日14时、20时及15日02时）（北京时,下同）模拟的福建省境内台风内核雨带和螺旋雨带逐时演变、台风路径与强度、逐时降水TS评分和空间相关差异显著,其中14日14时起报试验效果最好;而14日20时起报试验效果最差,这与该试验初始台风大风轴风速明显偏大有关;（4）在上述3个不同起报时刻试验基础上,分别增加双雷达反演风场资料的三维变分同化后,福建境内地面风场和台风内核雨带、螺旋雨带逐时分布、逐时降水TS评分和空间相关、台风环流结构以及U、V风垂直廓线分布均有明显改善,最大正影响时效可达24 h;但仅对1—6 h时效内台风路径有改善。      

Large-Eddy Simulation of Katabatic Flows

A large-eddy simulation model with rotated coordinates and an open boundary is used to simulate the characteristics of katabatic flows over simple terrain. Experiments examine the effects of cross winds on the development of the slope-flow boundary layer for a steep (20°) slope and the role of drainage winds in preventing turbulence collapse on a gentle slope (1°). For the steep flow cases, comparisons between model average boundary-layer velocity, temperature deficit, and turbulence kinetic energy budget terms and tower observations show reasonable agreement. Results for different cross slope winds show that as the cross slope winds increase, the slope flow deepens faster and behaves more like a weakly stratified, sheared boundary layer. Analysis of the momentum budget shows that near the surface the flow is maintained by a balance between downslope buoyancy forcing and vertical turbulence flux from surface drag. Above the downslope jet, the turbulence vertical momentum flux reverses sign and acceleration of the flow by buoyancy is controlled by horizontal advection of slower moving ambient air. The turbulence budget is dominated by a balance between shear production and eddy dissipation, however, buoyancy and pressure transport both are significant in reducing the strength of turbulence above the jet. Results from the gentle slope case show that even a slight terrain variation can lead to significant drainage winds. Comparison of the gentle slope case with a flat terrain simulation indicates that drainage winds can effectively prevent the formation of very stable boundary layers, at least near the top of sloping terrain.     

近海区域地面风场日变化的数值模似

本文用修改的一层σ坐标中尺度模式,模拟了我国部分沿海地区地面风场及其日变化,讨论了复杂地形和非绝热强迫效应对局地天气的影响。     

Causes of WRF surface energy fluxes biases in a stratocumulus region

In this study, we evaluate the ability of the Weather Research and Forecasting model to simulate surface energy fluxes in the southeast Pacific stratocumulus region. A total of 18 simulations is performed for the period of October to November 2008, with various combinations of boundary layer, microphysics, and cumulus schemes. Simulated surface energy fluxes are compared to those measured during VOCALS-REx. Using a process-based model evaluation, errors in surface fluxes are attributed to errors in cloud properties. Net surface flux errors are mostly traceable to errors in cloud liquid water path (LWP_cld), which produce biases in downward shortwave radiation. Two mechanisms controlling LWP_cld are diagnosed. One involves microphysics schemes, which control LWP_cld through the production of raindrops. The second mechanism involves boundary layer and cumulus schemes, which control moisture available for cloud by regulating boundary layer height. In this study, we demonstrate that when parameterizations are appropriately chosen, the stratocumulus deck and the related surface energy fluxes are reasonably well represented. In the most realistic experiments, the net surface flux is underestimated by about 10 W m^?2. This remaining low bias is due to a systematic overestimation of the total surface cooling due to sensible and latent heat fluxes in our simulations. There does not appear to be a single physical reason for this bias. Finally, our results also suggest that inaccurate representation of boundary layer height is an important factor limiting further gains in model realism.     

A model of atmospheric boundary-layer flow above an isolated two-dimensional ‘hill’; an example of flow above ‘gentle topography’

A numerical model is developed for two-dimensional turbulent boundary-layer flow above gentle topography — defined as not giving rise to mean flow separation. Although the model is formulated in a framework of mixing length and turbulent energy equation models for the surface layer of the atmospheric boundary layer, it could be modified to include higher-order closure hypotheses and/or extended to model gentle topography for the planetary boundary layer or on the sea bed. Results are presented for flow above a specific shape of hill and the effects of surface roughness and hill height are investigated.